Disc brakes are often used as brake systems for two-wheel vehicles such as motorcycles and bicycles. A disc brake includes a brake disc which is attached to a tire and which rotates together with the tire and brake pads that sandwiches the brake disc. Braking is accomplished by friction between the brake disc and the brake pads. The temperature of the brake disc is repeatedly increased to 500° C. or higher by the friction. Thus, brake discs are required to have high temper softening resistance, i.e., softening is not caused by exothermic heat during breaking, and the change of shape and galling are less likely to occur. For example, the appropriate range of the hardness of brake discs for motorcycles is usually about 30 to about 40 HRC and preferably 32 to 38 HRC on a Rockwell hardness scale C (HRC). Brake discs are required to maintain the hardness within the appropriate range even after they are repeatedly exposed to exothermic heat during braking. A hardness of less than the appropriate range can lead to a reduction in braking force due to the change of shape and galling of brake discs and can lead to cracking of brake discs. A hardness exceeding the appropriate range is liable to cause various problems such as brake squeals and reductions in braking force and pad life due to a reduction in the coefficient of friction. Furthermore, to ensure good appearance and a braking force, brake discs are also required to have corrosion resistance (rusting resistance). For the reasons described above, with respect to materials for brake discs, martensitic stainless steel sheets mainly containing 12% to 13% Cr are used. In particular, low-carbon martensitic stainless steel sheets containing 0.1% or less C are mainly used because an appropriate hardness is easily obtained only by a quenching process.
A lot of openings and chases are arranged in brake discs to improve cooling capability for exothermic heat during braking, eject wear debris and so forth, and achieve weight saving and good design. These openings and chases are formed by punching work or cutting work. So, a material to be worked is required to be soft. Thus, in the case where a brake disc is produced from a steel sheet, the steel sheet is annealed to adjust the hardness to 95 HRB or less on a Rockwell hardness scale B (HRB), formed into a disc shape, subjected to a quenching process in which the disc is held at 900° C. to 1100° C. for about 1 to about 10 minutes and then cooled to adjust the hardness to an appropriate range, and subjected to grind and rust-proofing to provide an end product. As the steel sheet described above, a steel sheet having a small extent of temper softening and a hardness of 30 HRC or more even when held at 500° C. for 60 minutes is used.
The recent improvement in the travelling performance of two-wheel vehicles has required further improvement in brake performance. There have been advances in the development of steel sheets, having higher temper softening resistance (heat resistance), for brake discs. For example, Japanese Unexamined Patent Application Publication Nos. 2001-220654, 2002-121656 and 2003-147491 and International Publication No. WO 02/18666 A1 disclose high-temperature heat-resistant steel sheets containing elements such as C, N, Nb, V, Cu, Ti, Mo, and B which have the effects of enhancing hardenability to obtain a stable as-quenched hardness and increasing temper softening resistance, the steel sheets having a hardness of 30 HRC or more even if tempering treatment is performed at a temperature exceeding 500° C.
The high-temperature heat-resistant steel sheets disclosed in JP '654, JP '656, JP '491 and WO '666 have relatively excellent heat resistance. In heat-resistance evaluation at 500° C. to 550° C. for about 60 minutes, good results are obtained. However, when the steel sheets were held in the temperature range described above for a longer time (for example, about 240 minutes), a sharp reduction in hardness and/or corrosion resistance was observed. That is, the results demonstrated that brake discs including these high-temperature heat-resistant steel sheets have insufficient corrosion resistance and stability of heat resistance when used for a long time (for example, about 240 minutes).
It could therefore be helpful to provide a steel sheet having stable corrosion resistance and the stability of heat resistance over a long period of use (for example, about 240 minutes) when the steel sheet is used as a brake-disc material. Specifically, it could be helpful to provide a steel sheet having hardenability such that the hardness falls within an appropriate range (32 to 40 HRC according to JIS Z2245) after quenching, the steel sheet after the quenching and after tempering having corrosion resistance such that the number of rust points is 4 or less after a salt spray test (SST) for 48 hours, and to provide a brake disc including the steel sheet. Furthermore, it could be helpful to provide a steel sheet having excellent temper softening resistance such that the hardness is in the range of 32 to 40 HRC after quenching, the hardness is in the range of 30 to 40 HRC after tempering treatment at 550° C. for 60 minutes, and the hardness is in the range of 28 to 40 HRC after tempering treatment at 550° C. for 240 minutes, the steel sheet after the quenching and after the tempering having corrosion resistance such that the number of rust points is 4 or less after a salt spray test (SST) for 48 hours, and to provide a brake disc including the steel sheet.